The present invention relates to an apparatus for ascertaining and/or keeping constant the mixing ratio of a liquid mixture, and more specifically but not exclusively for measuring and keeping constant the alcohol concentration of the dampening fluid used in offset lithography, comprising a float located in a vessel having a fluid supply inlet and an overflow connecting it with a fluid circuit, said float cooperating with a device worked in a way dependent on the level of the float in the fluid.
In the case of one known device of this sort for keeping constant the concentration of alcohol used in dampening liquid in an offset litho press, a valve is operated mechanically by the float, such valve then controlling a line for the supply of alcohol to be mixed with the fluid. Because of the high forces needed to open and close the valve in this case, which have to be provided by the float, it is not possible to keep the concentration constant within anything but very wide limits. A further drawback with this known device is that there is no possibility of indicating and recording or storing of the instantaneous alcohol concentration. Although proximity switches have been used as a remedy in conjunction with the float in order to control the valve for the pipe supplying the alcohol to be mixed with the dampening fluid, and in this case it was no longer necessary for the float to provide the forces needed for opening and shutting the valve, it was still not possible for the instantaneous alcohol concentration values to be indicated and stored. Furthermore, since the proximity sensors have to be mechanically secured in place the set point of the system may not be adjusted and remains at a certain value, unless a complex operation is performed to adjust the position of the proximity sensors.